1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as copiers, printers, facsimiles and combined machines thereof as well as to an image forming method using the image forming apparatus. More particularly, the invention relates to an image forming apparatus and an image forming method thereof, the image forming apparatus including: a developing device that forms a toner image by using a development roller to advance to an image carrier a developer including a toner and a carrier and by using a voltage applicator to apply a developing bias between the development roller and the image carrier for supplying the toner onto the image carrier; a transfer device for transferring the toner image formed on the image carrier to a transfer receiving material; a toner recovery device for recovering the toner remaining on a surface of the image carrier after toner image transfer; and a transport mechanism for transporting to the developing device the toner recovered by the toner recovery device. In the image forming apparatus and method, the invention seeks to prevent wasteful discarding of the toner in the developer, which may encounter carrier deterioration while stored in the developing device.
2. Description of Related Art
It is a general practice in the electrophotographic image forming apparatus such as copiers, printers, facsimiles and combined machines thereof that the developing device supplies the toner to the image carrier with an electrostatic latent image formed thereon so as to form thereon a toner image corresponding to the electrostatic latent image. The transfer device transfers the toner image formed on the image carrier onto the transfer receiving material such as a recording medium or an intermediate transfer belt. Subsequently, the toner recovery device recovers from the surface of the image carrier the toner remaining after the toner image transfer. There is known an apparatus which employs returning means for returning the toner recovered by the toner recovery device to the developing device for the purpose of effective use of the recovered toner.
The above developing device includes a single component development device using a toner alone as the developer, and a two-component development device using a developer including the toner and the carrier. The two-component developing device using the developer including the toner and the carrier is superior in terms of quick and adequate toner charging for high-speed image formation.
In such a developing device of the two-component development system, the developer including the toner and the carrier is agitated to charge the toner in the developer by contact with the carrier. The developer with the charged toner is advanced to the image carrier by the development roller. The voltage applicator applies the developing bias between the development roller and the image carrier so as to supply the toner to the image carrier. Thus, the toner image corresponding to the electrostatic latent image is formed on the surface of the image carrier.
In a case where the developer is decreased in toner content as a result of the above-described toner supply to the image carrier, a toner replenisher replenishes the developing device with the toner.
The above developing device of the two-component development system has the following problem. While the developer is used over a long period of time, the carrier in the developer is gradually deteriorated so as to become incapable of adequately charging the toner. Accordingly, the developing device cannot accomplish proper image formation.
It has been a conventional practice to remove the developer with the deteriorated carrier from the developing device for replacement with a fresh developer or to replace the developing device per se with a new one.
However, in the case where the developer with the deteriorated carrier is removed from the developing device for replacement with a new one, or where the developing device itself is replaced with a new one, the developer actually contains a substantial amount of toner that is still usable. Namely, there is a problem that the usable toner in the developer is wastefully discarded.
More recently, there has been proposed an apparatus wherein the above developing device is equipped with separation means for separating the toner from the developer being recovered, as disclosed in Patent Document 1 (Japanese Patent Publication No. 3581720). The separation means includes a developer recovery portion and a toner recover portion and applies to a developer recovery sleeve a charge of the opposite polarity to that of the toner so as to separate the toner from the developer being recovered. The separated toner is returned to the developing device for recycling.
However, the constitution disclosed in Patent Document 1 requires the developing device to be equipped with many devices, resulting in cost increase. Furthermore, the developing device is increased in size.